In U.S. Pat. No. 4,121,109 there is disclosed an electron accelerator intended for use in radio therapy. To generate X radiation in this electron accelerator, a target is exposed to the electron beam issuing from the acceleration tube. Behind the target, in beam direction or in the path thereof, there are arranged an electron absorber, in which the remaining electrons are filtered out of the X radiation, and a collimator with a passage aperture for masking out the maximun, usually conical X-ray field being used. A compensation body is positioned in the beam passage aperture of the collimator by which the dosage output of the issuing X radiation is equalized over its entire cross-section. In such electron accelerators there is a disadvantage however that, in addition to the therapeutically desired roentgen quanta, neutrons also are produced which increase the radiation load of the patient undesirably.